Excretion – Class 11 | Chapter – 19 | Biology Short Notes Series PDF

Excretion, the biological process by which an organism rid themselves of the excess waste products and the nitrogenous by-products of metabolism through urethra. Moreover, through this process, organisms control osmotic pressure – the balance between inorganic ions and water – and maintain acid-base balance. In this way, homeostasis is promoted, the constancy of the organism’s internal environment.

Excretion is an essential process and is common in all living organisms, including the animals, plants, insects, birds and even unicellular organisms.

The nature of nitrogenous waste (Urea, Ammonia, Uric Acid) formed and their excretion waste varies among animals (mammals, aves, fishes, amphibians, insects).

The excretory wastes are generated as the by- products of metabolic processes. 

Importance of Excretion

  • Regulation of blood ionic composition
  • Control of blood pH
  • Regulation of blood volume and blood pressure
  • Maintenance of blood osmolarity
  • Excretion of waste and foreign substances
  • Maintenance of osmoregulation

Types of Excretory Wastes

The excretion also takes place in plants- during photosynthesis, Oxygen and water are excreted, as they are waste for plants at that time and require removal. The Rubber trees exude resins and latex in the process of excretion.

Organism Excretory Waste Process Toxicity
Bony Fishes, Aquatic Amphibians, Aquatic Insects Ammonia Ammonotelism Highest and most soluble
Mammals, Marine Fishes, Terrestrial Amphibians Urea Ureotelism Less toxic and less soluble
Reptiles, Birds, Insects, Land Snails Uric Acid Uricotelism Average toxicity, in form of pellets or paste with minimum water loss

Modes of Excretion

Based on the excretory product, five modes of excretion are known in animals. They are:

  • Ammonotelism (Type of excretion- ammonia)
  • Ureotelism (Type of excretion – urea)
  • Uricotelism (Type of excretion – uric acid)
  • Aminotelism (Type of excretion – amino acids)
  • Guanotelism (Type of excretion – guanine)


The process of eliminating ammonia from the body is known as ammonotelism, and the organisms which exhibit this nature are called ammonotelic. Most fish, protozoans, echinoderms, poriferans and crustaceans fall into this category. Aquatic animals excrete ammonia directly into the environment; where the compound is quickly diluted. It is also very toxic to tissues.


In some mammals and amphibians, urea is excreted as a metabolic waste product. Such organisms are called ureotelic. In these organisms, ammonia that is produced is converted to urea in the liver of animals and is released back into the blood. The kidneys filter the urea and expel the urea outside the body. Some of the urea is retained in the matrix of the kidney to maintain a desired osmolarity in the organisms. Humans are ureotelic as we expel the urea through urine. Moreover, urea is comparatively less toxic than ammonia.


Uricotelic animals remove nitrogenous wastes as uric acid in the form of pellets or paste. Metabolically, this process is quite costly; however, the water loss is minimal, and it is the least toxic. Moreover, since uric acid is not readily soluble in water, the excrements form pasty white suspensions. Most reptiles, birds, and insects are classified as uricotelics.


Certain molluscs and echinoderms excrete excess amino acids. This feature is called aminotelism.


Spiders convert the ammonia into guanine before excretion. This characteristic is also found in some reptiles, birds and earthworms. It is also insoluble in water; hence no water is required for its excretion.

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